This invention relates to high power fiber lasers and more particularly to a method and apparatus for pumping the fiber laser.
Fiber lasers exhibit great potential for applications as high power directed energy sources. Fiber lasers offer the advantages of high efficiency, minimal cooling requirements, and good beam quality. However, the main problem with fiber lasers is obtaining high power out of a fiber because it requires a significant amount of pumping power. Even with the larger cores available through the use of a double clad system, it is still only with difficulty that one can get a few hundred watts of pump light into the end of a fiber. For 1-KW applications one needs at least 2 to 4 KW of laser pump power. How to obtain such high pump power for solid-state lasers has proved to be a problem.
Recent laser demonstrations of Yb doped fibers have shown the power scaling capability of fiber lasers with  greater than 100W single mode output and beam quality of M2≅1.1. However power scaling beyond the 200W level is severely limited by current fiber and diode pump coupling technology of end-pumped, single-mode double clad Yb:fiber lasers. Step-index, single mode Yb:fiber lasers operating around 1 xcexcm are limited to about a 200 W power level due to the onset of non-linear optical effects that lead to damage of the fiber core as well as defect related damage at the high CW intensities encountered ( greater than  greater than 100 MW/cm2). Pump power coupled into the two ends of a double clad fiber laser is limited to about 500 W, employing the brightest currently available laser diode arrays. Even with the high efficiency (50-60%) of an Yb:fiber laser this would result in a maximum fiber laser output of only 250-300 W.
Advances in side coupling to fibers now permit the coupling of multiple discrete emitters to a fiber. However at the pump powers required for a 1-KW laser this would represent greater than 1000 individual diode packages and pump fibers, resulting in a very complex system.
What is required is a 1-KW fiber laser generating a single mode, polarizing preserving output using a longer operating wavelength to achieve a diffraction-limited output. Furthermore, one needs to dramatically reduce the complexity of ultra-high-power fiber lasers.
By way of further background, fiber lasers show great promise as efficient high power continuous wave laser sources. They are highly efficient due a combination of low loss and long interaction length. They can produce diffraction-limited single mode outputs, have a very high surface to volume ratio to efficiently dissipate heat and can use all-fiber couplers and reflectors developed for telecom applications to achieve monolithic, alignment-free resonant cavities.
The highest power yet reported is 110 W from a CW Yb:fiber laser operating in a near-diffraction limited spatial mode with an optical efficiency of 58% and a wall plug electrical-to-optical efficiency of  greater than 20%. Work in progress has demonstrated greater than 200 W single-mode CW output with prospects for 300-500 W in multimode operation. In pulsed operation, 64-KW peak power with 51.2 W average power was generated by amplifying 10 ps 1064 nm pulses from a mode-locked Nd:YVO4 laser in a large mode area (LMA) Yb:fiber amplifier.
However, several important limitations exist to scaling the output power of fiber lasers. These include most notables limited pump coupling to the fiber.
Thus, the principal limitation to date has been the coupling of large pump powers into the active regions of a single mode fiber. The advent of double-clad geometry fiber lasers, where the pump light is guided in a second multimode core, led to greatly increased output power. However, the amount of pump power that may be coupled into the end of a double clad fiber is limited by the brightness of the laser diode and the two entry ports. Newer, highly collimated laser diode arrays have achieved coupling of 250 W into a 400 xcexcm, 0.22 NA fiber which could permit the end-coupling of up to 500 W of pump into the fiber. However this is still several times less power than that required for a 1-KW output.
The other primary limitation to producing high power fiber lasers is the onset of non-linear optical effects. More benign non-linear affects such as Stimulated Raman Scattering (SRS) and Self-phase modulation (SPM) result in spectral broadening. However Stimulated Brillouin Scattering (SBS) can lead to self-Q switching of the fiber which in turn can damage the fiber due to high intensity. The role of thermal effects has not yet been shown theoretically beyond tuning of the laser output with temperature. However, at high average powers, thermal effects may significantly affect laser performance through thermally induced changes in the refractive index profile and thermal population of the terminal laser level in quasi-three lasers such as Yb, Er and Tm. Imperfections in the fiber surface can also lead to damage due to the high CW intensities ( greater than 100 MW/cm2 for 100 W output).
Although fiber lasers have numerous advantages over bulk lasers, one of their primary disadvantages is the difficulty of pumping them due to their small dimensions. Hence, to date fiber laser CW power output has been limited by the amount of pump power that can be coupled into the ends of the fiber.
A significant advance in high power fibers lasers was the advent of the double clad fiber structure. The central core region of radius, r, and an index, n2 contains an active laser ion (such as Nd, Yb, Er, or Tm) and is usually sized to support only the fundamental mode. It is surrounded by an inner cladding region of index, no and usually has a polygonal cross-section. This is in turn surrounded by the outer cladding with index, n0. For guiding in each region the indices must meet the condition n0 less than n1 less than n2. The numerical aperture, NA of each region is given by:
NA=(ni2xe2x88x92nj 2)1/2
The principle of operation of this structure is that light from low brightness pump sources can be guided in the inner cladding region and can be absorbed by the active core, eliminating the need for high brightness sources to pump directly into the active core. Since the overlap area between the inner cladding and the core is small, then the effective pump absorption, xcex1eff is the core absorption coefficient, xcex1 reduced by the ratio of the areas:
xcex1hd eff=xcex1Acore/Aclad
Furthermore if the inner cladding region is circularly symmetric and the core is centered, the absorption is further reduced since only the meridonal rays intersect the core region. The use of a polygonal cladding eliminates skew rays, whereas offsetting the core allows skewed rays to access the core region.
The amount of power that may be coupled into the ends of a double clad fiber laser is determined by the brightness of the pump source and the size and numerical aperture of the inner cladding. The radiance theorem states that in radiance (diameter-NA product) is constant in an optical system expressed as the following relation:
DoutNAout=DinNAin
Currently the best results for fiber coupling of laser diodes is 250 W from a 400 xcexcm, 0.22 NA fiber. This would be sufficient for coupling into the inner cladding region of a typical double clad fiber (200 xcexcm, 0.44 NA). Using both ends, up-to 500 W of pump could be coupled. However this is still several times less pump than that required for a 1-KW laser source. Given these limitations, it is clear that in order to scale fiber lasers to kilowatt output powers, a scalable side-coupled pump technique is required.
As to side coupled pumping schemes, there are three potential scalable coupling techniques, presented here in order of increasing risk (i.e., difficulty related to fabrication/implementation). As the risk increases, however, the final system complexity generally decreases. Note that for each technique presented below, each is capable of delivering 20-30W of pump power per tap into a double clad fiber (DCF).
The first method is a side coupled pump scheme which is key for the realization of high power fiber lasers since it permits the coupling of an arbitrary number of pump lasers into the active core of the fiber laser. In this scheme, a multimode pump fiber is fused at a grazing angle into the inner cladding of a double clad fiber laser. Since the angle of the coupling is small, pump light already captured by the inner cladding from another pump region remains within the double clad structure. Discrete 100 xcexcm broad area emitters are coupled into multimode fiber pigtails which are then fused together and spliced into the multimode fiber taps. The broad area emitters have proven reliability and the use of multiple packages effectively distributes the heat load. However the high package count is too cumbersome for a KW class fiber laser.
The second method is a modification of the above side coupled pump scheme employing commercially available fiber array packages (FAPs) instead of single emitters. Fibers in the FAP are fused together and then fused into the double clad fiber coupler. This is as reliable and efficient as the first method, with fewer parts and less cost. The laser diode heat load is more localized, but this is easily offset by the space saved by using diode bars rather than single emitters.
A third method, and one which is the subject of this patent, uses a modified V-groove approach to directly couple a stack of at least 5 emitters either directly into the double clad fiber, or into a pigtail that can be fused onto the double clad fiber. In the subject method a number of emitters are coupled to successive V-grooves in the inner cladding of a double clad fiber. The grooves are configured such that all pumping light introduced into the inner cladding is reflected in only one direction down the fiber, with no backwardly reflected radiation. The angles on the facets of the V-grooves are such as to assure total internal reflection. This means that as may sources as desired can be coupled to the fiber so as to raise the cumulative pumping power to in excess of 2-KW. This in turn results in a fiber laser output exceeding 1-KW, useful for industrial as well as military applications. This method dramatically reduces the cost, parts count, and complexity of the system.
Thus, in the subject invention, a diode array modified V-groove coupling is used so that multiple emitters can be arrayed along a fiber to provide the required pumping power.
This is a direct diode-coupling scheme based on V-groove side-pumping geometry. In the subject invention, a modified V-groove technology allows the direct side coupling of several emitters within a several millimeter length of fiber, whereas conventional V-groove technology permits only one emitter to be coupled per absorption length of fiber, about 2 meters given an absorption coefficient of 4.6 dB/m. As such, the pump density using conventional V-groove coupling is significantly lower. In one embodiment, a longer operating wavelength is obtained by using thulium as the active lasing ion, resulting in tunable laser action from 1.8-2.1 xcexcm. This high power 2 xcexcm laser technology has important dual-use applications in IR countermeasures, medicine and materials processing made possible by an efficient and scalable direct side-coupled pump scheme.
What is provided is a scalable high power fiber laser pump module that has a tenfold decrease in complexity over existing side-pumping techniques. This is achieved by the integration of discrete single emitters into custom laser diode bar packages and the use of the subject side-coupled fiber pump technique using a modified xe2x80x9cV-groovexe2x80x9d structure that permits multiple emitters to be directly coupled into the pump clad region of a double clad fiber laser.
V-groove coupling geometry was first presented by Ripin and Goldberg Elec. Lett., vol. 31, p. 2204 (1995). While slightly modified geometries have been presented as reported by Goldberg, CLEO 2000, p. 572, all rely on the same basic concept. A small region of the double clad fiber is stripped to reveal the inner cladding. A 90xc2x0 notch is cut into the side of the inner cladding, providing a total internal reflection, TIR, surface for diode light focused onto one side (alpha facet) of the notch. Assuming a refractive index of xcx9c1.5 and an NA of 0.44 for the double-clad portion of the fiber, the critical angle for confinement is 73xc2x0 with respect to the normal to the fiber surface. Reflection off the notch will only occur if the diode light is incident at an angle of 42xc2x0 or larger relative to the normal of the notch, assuming that air (n=1) is on the opposite side to the notch. Alternately, light incident on the notch at angles larger than 62xc2x0 will reflect off the notch but will then be incident on the fiber walls at angles less than the critical angle and will not be confined in the 0.44 NA fiber. Hence, the light cone of the pump laser must be 20xc2x0 or less, and incident on the notch at an angle of 52xc2x0 for efficient coupling.
In one embodiment, for V-groove side-pump coupling, a small section of fiber with the outer cladding stripped has a notch, or groove, fixed on the side. Diode light is focused onto this facet and, if properly conditioned the diode light is coupled into the double clad fiber.
Typical V-groove features are about 20-50 xcexcm deep in fibers with inner cladding diameters of 120-180 xcexcm. The diode light is typically conditioned so that the fast axis is focused onto the V-groove in a 15xc2x0 light cone.
While a single V-groove is excellent at coupling pump light into a fiber core, attempting to use multiple V-grooves fails due to light escaping because critical angles are exceeded.
The subject modified V-groove approach avoids problems associated with multiple conventions V-groove. In the subject invention a second groove is successfully placed closely following the first V-groove. If the second groove were also at 45xc2x0, it would directly out-couple a significant portion of the pump light from the first V-groove. Instead, a shallow facet, referred to as the beta facet, since it is formed at an angel xcex2 with respect to the fiber axis, gradually transitions the fiber between V-grooves to keep the light confined. Since some of the light that comes off the first V-groove referred to as the alpha facet is now incident on the beta facet, it could spoil the NA, numerical aperture, of the pump light. For the two most extreme rays behave under this geometry (assuming the same conditioning of the diode light incident on the V-groove), one ray is xe2x88x923xc2x0 off normal to the fiber axis. This results in a 42xc2x0 angle of incidence at the alpha facet fiber-to-air interface and matches the critical angle for total internal reflection. Another ray is 11xc2x0 off normal to the fiber axis, with this ray forming the 14xc2x0 diode-focusing angle. Upon reflection at the alpha facet it is incident on the beta facet at an angle of 79xc2x0-xcex2. When the ray then hits the flat side of the fiber, it is well within the required 42xc2x0 incidence angle for total internal reflection at the fiber-to-air interface. However, in one embodiment, this ray is to be incident at the 73xc2x0 angle (xcex8c) required for total internal reflection in the 0.44 NA fiber. This requirement results in a facet angle of xcex2=3xc2x0 or less for confinement in the double clad fiber. The beta facet then terminates at a second alpha facet useful for injecting a second pump source.
In this manner any number of sources can inject energy into the inner core and have the energy propagate down the filer without loss due to a subsequent V-notch and without exiting the fiber. Thus enough energy can be pumped into the core to yield a 1-KW output or better.
Note that the addition of a second alpha facet does not increase the NA of the pump light beyond the 0.44 required by the double clad fiber.
Note also that, as will be demonstrated, the addition of four more alpha facets represents a five-fold increase in pump density over conventional V-groove coupling. Assuming 200 xcexcm-wide diode emitters can be coupled with this geometry, approximately xcx9c30W of pump can be delivered to the fiber. This modified V-groove can be applied directly to the double clad fiber or to a pigtail that is then spliced into the fiber laser.
In summary, a modified V-groove structure in a double clad laser system permits multiple emitter side pumping of a fiber laser. In one embodiment, a stack of at least five emitters have individual outputs coupled into the inner cladding, with as many sources as desired coupled to the fiber to raise the cumulative pumping power to in excess of 2-KW, thus to achieve a 1-KW fiber laser.